Improving Control System Effectiveness for Ducted Fan Vtol Uavs Operating in Crosswinds

A valuable new resource being developed for today’s soldier are small ducted fan VTOL UAVs. Although beneficial in many ways, a major operational problem of ducted fan vehicles is precise control when flying in crosswinds and turbulent conditions in general. There are two significant, inherent issues associated with ducted fan control in crosswinds; 1) lateral momentum drag and 2) a duct stabilizing torque which resist tipping into the wind. For vehicles designed for flight over large ranges in angle of attack (i.e. “transition to high-speed cruise”), trim over the flight envelope is strongly affected by the duct moment characteristics. Poor CG placement can lead to control saturation at intermediate flight speeds. These are characteristics of all ducted fan vehicles designed to hover as well as transition to horizontal flight. Techsburg, in collaboration with AVID LLC. and Honeywell Labs, has recently completed a research program to investigate and test new aerodynamic control devices to improve the handling of small ducted fan UAVs in crosswinds. This project consisted of both computational analysis and modeling of the vehicle aerodynamics and control system, as well as wind tunnel experiments using a powered ducted fan VTOL UAV model. This work leveraged methodologies and testing resources developed as part of a larger DARPAfunded VTOL UAV program. The research focused on a better understanding of the baseline vehicle and control vane aerodynamics, and selection and aerodynamic modeling and testing of new VTOL UAV auxiliary control device concepts. The analytical results are then used as input into an existing control system modeling framework, which in turn can be used to predict the vehicle’s dynamic response to turbulence. In addition to an improved flight control system, insights into establishing disturbance rejection criteria for VTOL UAVs are sought for use in future vehicle development efforts. This paper focuses on describing the baseline vehicle aerodynamics in a crosswind, as well as characteristics and design considerations for traditional control vanes used for this type of vehicle. Some limited, initial results from analysis of auxiliary control devices are presented as well.